1. Field of the Invention
The present invention relates to a liquid jetting apparatus such as an ink jet printing apparatus having an air discharge mechanism which discharges air existing in a liquid to be supplied to the jetting head, and to a liquid supply unit of the liquid jetting apparatus.
2. Description of the Related Art
As an ink-jet printing apparatus which is an example of a liquid jetting apparatus, a printing apparatus in which a so-called tube-supply method is adopted has hitherto been known. In this case, an ink is supplied to a jetting head, which reciprocates while facing or be opposite to a recording paper, from an ink cartridge provided in the body of the ink-jet printing apparatus (apparatus body), via a flexible ink supply tube. In such a printing apparatus, it is required to make the capacity or volume of the ink cartridge be large or increased, and to make the size of the apparatus be compact. The means for achieving the large capacity and compactness can be exemplified by making the size of a liquid supply unit be small, the liquid supply unit including a jetting head, reciprocating integrally with the jetting head and also guiding the ink to the jetting head from or via the ink supply tube.
Note that when, in particular, the frequency of use of the printing apparatus is low, it has been known that air (an air bubble) is developed or grown in a channel (flow passage) which guides the ink from the ink cartridge to the jetting head. When this air enters into the jetting head, there is a possibility that desired jetting characteristics might not be achieved. To address such a situation, when an air storage portion which traps air is provided on the channel of the liquid supply unit at an intermediate portion of the channel, it is possible to prevent the air from entering into the jetting head. However, when the capacity of the air storage portion is made great or substantial in order to trap more air, it is difficult to make the size of the liquid supply unit be small. Therefore, a mechanism has been proposed in which an air storage portion having a small capacity is provided on the liquid supply unit, and the air is discharged forcibly from the air storage portion at a predetermined timing (see Japanese Patent Application Laid-open No. 2007-175996).
Japanese Patent Application Laid-open No. 2007-175996 discloses a structure including a liquid supply unit having an air storage portion which is closed normally by biasing a valve element (valve member) toward a direction of closing by a coil spring; and a valve rod which is provided at a predetermined standby position on a side of the body of the printing apparatus. Further, in a case that the air inside the air storage portion is to be discharged, a carriage which integrally accommodates the jetting head and the liquid supply unit is arranged (made to be located) at the standby position, and the valve element of the liquid supply unit is pressed by the valve rod, thereby opening the valve element resisting against the biasing force of the coil spring. Furthermore, a negative pressure is generated by a pump provided on the side of the apparatus body, and the air inside the air storage portion is discharged.
However, in the structure described in Japanese Patent Application Laid-open No. 2007-175996, the size of the valve element and the size of the valve rod are comparatively small. Accordingly, for pressing or pushing the valve element with the valve rod, it is necessary to position the liquid supply unit to the standby position highly accurately. Further, at the time of discharging air, the pump on the side of the apparatus body is connected to the air storage portion on a side of the liquid supply unit. On the other hand, at the time of printing, the pump on the side of the apparatus body is separated (is disconnected) from the air storage portion on the side of the liquid supply unit. Therefore, upon discharging the air, it is necessary to secure the air-tightness of the air discharge channel at a location at which apparatus body and the liquid supply unit are connected. Furthermore, in addition to the pump, it is necessary to separately provide, on the apparatus body, a valve-element opening mechanism which includes the valve rod, corresponding to a standby position of the liquid supply unit. Therefore, it is difficult to realize the small size of the printing apparatus.